1. Field of the Invention
This invention relates to alignment layers for fabricating liquid crystal displays (LCDs).
2. Description of the Related Art
Liquid crystals are useful for electronic displays because polarized light traveling through a liquid crystal layer is affected by the layer's birefringence, which can be changed by the application of a voltage across the layer. By using this effect, the transmission or reflection of light from an external source, including ambient light, can be controlled with much less power than is required for the luminescent materials used in other types of displays. As a result, liquid crystal display (LCD) devices have become increasingly important in displays which require very low consumption of electrical power or where the environment dictates a lightweight, planar, flat surface. Thus, LCD's are used in display devices such as wristwatches, pocket and personal computers, calculators, aircraft cockpit displays, etc. These applications highlight some of the advantages of LCD technology including very long operational life in combination with very low weight and low power consumption. (For an overview on LCD technology, see U.S. Pat. No. 5,612,801).
In its simplest form, a typical LCD stack comprises a liquid crystal cell, a polarizer and/or an analyzer layer. Additionally the LCD stack often includes compensator films which improve the contrast and viewing angle of the display. The use of such compensators have been disclosed in U.S. Pat. No. 5,196,953 (Yeh et al.), U.S. Pat. No. 5,504,603 (Winker et al.), U.S. Pat. No. 5,557,434 (Winker et al.), U.S. Pat. No. 5,589,963 (Gunning, III et al.), U.S. Pat. No. 5,619,352 (Koch et al.), U.S. Pat. No. 5,612,801 (Winker) and U.S. Pat. No. 5,638,197 (Gunning, III et al.).
Compensators can be placed in a variety of locations between an LCD's polarizer and analyzer layers. With the addition of one or more compensator layers, the contrast ratio is improved over a wide range of viewing angles by careful orientation of the optic axis of the compensator, since the combined retardation effect of the compensator cancels the phase retardation inherent in liquid crystal displays.
FIG. 1 depicts a coordinate system which is used to describe the orientation of both liquid crystal and birefringent compensator optic axes. Light 116 propagates toward the viewer 100 in the positive z direction 102 which, together with the x-axis 104 and the y-axis 106, forms a right-handed coordinate system. The polar tilt angle Θ 108 is defined as the angle between the liquid crystal molecular optic axis (c-axis) 110 and the x-y plane, measured from the x-y plane. The azimuthal angle Φ 112 is measured from the x-axis to the projection 114 of the optic axis onto the x-y plane.
Two types of compensators which are well known are O-plate and A-plate compensators. An O-plate is a birefringent layer with its optic axis, on average, oriented at a substantially oblique angle with respect to the plane of the display. “Substantially obliques” implies an angle appreciably greater than 0° and less than 90°. An A-plate is a birefringent layer with its optic axis oriented parallel to the plane of the display. Additionally, the optic axis of both A-plates and O-plates is aligned, on average, at a certain angle referred to as the azimuthal angle. (see FIG. 1). A compensator's optic axis may be oriented by the use of alignment layers. The use of rubbed polymer films, i.e. alignment direction and tilt angle controlling films, dominates the process technology used in the production of all categories of liquid crystal displays, and polyimides are the most common alignment films in use today. (The alignment of the polymer film itself is induced by gentle buffing or rubbing with specific cloths.)
Unfortunately, polyimides are expensive, and can cause defects when cast onto an already aligned compensator layer. As such, fabrication of compensators often requires the multilayer coating and transferring of films by lamination using pressure sensitive adhesives. This process involves coating a substrate (such as glass) with an alignment layer, then the compensator layer to induce orientation of the compensator's optic axis, and laminating the aligned compensator onto the LCD or compensator stack, thus separating it from the alignment layer. Additional compensators can similarly be added onto the compensator stack by lamination. Since the films to be transferred are thin (e.g. about 1μ) and have a large area (e.g. about 43×43 cm), lamination of such films can cause defects such as stress lines. Consequently, fabrication requiring multilayer film transfer lamination is inefficient, labor intensive, and expensive.
A-plates may also be fabricated by the use of uniaxially stretched polymer films, such as polyvinyl alcohol, or other suitably oriented organic birefringent materials. Unfortunately, the quality and uniformity of the optical properties of such prior art A-plates are generally poor. Furthermore, the A-plate must be held under tension and the material must be relatively thick to obtain sufficient anisotropy. The lamination of such A-plates with other retardation films often results in stress-induced birefringence when exposed to elevated temperatures. (see U.S. Pat. No. 5,995,184).
In addition to its use as an alignment layer for compensator films, polyimide is also used as an alignment layer in the liquid crystal cell itself.